1. Field of the Invention
The present invention relates generally to a semiconductor device used for an IC card and an RFID (Radio Frequency Identification: contactless automatic identification technique by use of radio frequencies). In particular, the invention relates to a semiconductor device including a contactless IC card and an RFID chip which receive power and data from an external device in a contactless manner and transmit data to the external device, and a driving method thereof.
Note that the RFID chip is used, for example, as a tag. In addition, the IC card is one type of the RFID chip.
2. Description of the Related Art
In recent years, IC chips such as contactless ID chips using radio frequencies, namely those called RFID chips are attracting attentions, and achievement of higher performance thereof is expected. RFID chips have advantages that recoded data can be read out in a contactless manner, batteryless operation is possible and excellent durability, weather resistance and the like are provided.
In addition, the RFID chips can incorporate functional circuits such as a CPU. That is, the RFID chips can incorporate logic circuits for security management and the like as well as functioning as a data recording medium. The RFID chip has various applications such as personal identification, product identification and position measurement.
Conventional RFID chips have a configuration as shown in FIG. 2. An RFID chip 217 shown in FIG. 2 comprises a power supply circuit 214, an input/output circuit 215, an antenna circuit 216, a logic circuit 210, an amplifier 211, a clock generator circuit/decoder 212, a memory 213 and the like. The antenna circuit 216 comprises an antenna wiring 201 and an antenna capacitor 202.
The RFID chip 217 does not have its own power supply, and instead, it operates with power supplied through reception of a radio wave 218 generated by an RF reader/writer 200.
The operation of the RFID chip 217 is described with reference to FIG. 2. When the antenna circuit 216 receives the radio wave 218 from the RF reader/writer 200, it is detected as an input signal by the input/output circuit 215 which comprises a first capacitor 203, first and third diodes 204 and 207, a third capacitor 208, a switch element 209 and the like. The signal is once amplified to have a sufficiently large amplitude by the amplifier 211 before being split into a clock, data and command by the clock generator circuit/decoder 212. The transmitted command is then decoded in the logic circuit 210, thereby data is read from/written to the memory 213.
The data reading is carried out by turning ON/OFF the switch element 209 using an output of the logic circuit 210. Accordingly, impedance of the antenna circuit 216 is changed, which in turn changes reflectivity of the antenna circuit 216. The RF reader/writer 200 reads out data from the RFID chip 217 by monitoring the change in reflectivity of the antenna circuit 216.
Power consumed in each circuit of the RFID chip 217 is supplied by a DC power supply VDD which is generated by detecting and smoothing the radio wave 218 which is received by the antenna circuit 216, in the power supply circuit 214. The power supply circuit 214 comprises the first diode 204, a second diode 205 and a second capacitor 206. The second capacitor 206 has a sufficiently large capacitance value in order to supply power to each circuit.
FIGS. 11A and 11B illustrate an output (B) of a DC power supply outputted from the power supply circuit 214 relatively to an antenna input signal (A) received by the antenna circuit 216. Negative components of the antenna input signal are removed by the first diode 204 and the second diode 205, and only positive components thereof are supplied to each circuit through the second diode 205. The capacitor 206 stores positive components which have passed through the second diode 205, and supplies power when an antenna input signal is negative. Therefore, the VDD has substantially a constant value, and thus the power supply circuit 214 functions as a DC voltage source.
The following Patent Document 1 discloses an example of such circuit.
[Patent Document 1] Japanese Patent Laid-Open 2000-299440
FIG. 3 illustrates an antenna circuit 308 and a power supply circuit 307 which are the partial components of an RFID chip 309. The antenna circuit 308 comprises an antenna wiring 301 and an antenna capacitor 302. The power supply circuit 307 comprises a first capacitor 303, a first diode 304, a second diode 305 and a second capacitor 306.
The RFID chip has a property of an batteryless operation, and it has a mechanism that circuits incorporated in the RFID chip operate with a DC voltage which is generated by receiving a radio wave from an RF reader/writer in the antenna circuit 308 and rectifying it in the power supply circuit 307.
FIG. 12 illustrates a relationship of the intensity of an electromagnetic field (effective value) received by the antenna circuit 308 and the intensity of a DC voltage rectified by the power supply circuit 307. As shown in FIG. 12, the intensity of the DC voltage rectified by the power supply circuit 307 is determined approximately proportionate to the intensity of the original electromagnetic field. Therefore, in the case where the antenna circuit 308 is exposed to a high electromagnetic field, a high AC voltage signal is generated in the antenna circuit 308. As a result, a DC voltage obtained through rectification of the AC voltage in the power supply circuit 307 is also high.
Accordingly, a high voltage is applied to a memory, a clock generator circuit and the like in the logic circuit portion, and in such a case, the logic circuit portion might generate heat. Otherwise, circuit elements thereof might be broken by the high voltage or other problems might arise.
In view of the foregoing problems, it is an object of the invention to prevent generation of a high voltage even when a high electromagnetic field is applied, and thus prevent a heating circuit and element breakdown.